Solid-phase bonding of metals



Aug. 2, 1960 H. R. PFLUMM ETAL SOLID-PHASE BONDING 0F METALS Filed Oct. 30. 1957 FIG-2'.

United States PatentO SOLID-PHASE BONDING OF METALS Heinz R. Pflnmm, North Attleboro, and Freeman P. .Rogers, Plainville, Mass., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Oct. 30, 1957, Ser. No. 693,283 Claims. (31. 29-4743 This invention relates to the solid-phase bonding of malleable metals, i.e., the joining of the same or different solid malleable metals in the forms of coaxial cores and shells Without adding or otherwise producing a liquidphase material between them, being an improvement upon the process set forth in our United States patent application Serial No. 612,635, filed September 28, 1956, entitled'Solid-Phase Bonding of Metals eventuated as Patent No. 2,834,102, pertinent information in which is incorporated herein by reference.

Among the several objects of the invention may be noted the provision of improvements over the process set forth in said application, adapted to be conveniently carried out at lower cost for producing cored wires, rods, tubes and the like, wherein a solid-phase bond is attained between the core and a surrounding sleeve; the provision of an improved process of the class described wherein one of the metals is initially in the form of a rod or the like of any convenient cross section and the other is initially in the form of a sleeve of similar cross section, advantage being taken of such related forms conveniently to obtain a combined heating and pressing action by simple heating; and the provision of a method of the class described which is applicable to the bonding of any malleable metals. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the steps and sequence of steps, and features of manipulation, which will be exemplified in the methods hereinafter described, and the scope of which will be indicated in the following claims.

-In the accompanying drawings, in which several of various possible embodiments of the invention are illustrated,

Fig. 1 is a sectioned isometric view illustrating features of one form of the invention;

Fig. 2 is a fragmentary axial section illustrating a modification of the form of the invention shown in Fig. 1;

Fig. 3 is a sectioned isometric view illustrating certain features of a second form of the invention; and,

Fig. 4 is a fragmentary axial section illustrating a modification of the form ofthe inventionshown in Fig. 3.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

As used hereinafter, the term metals includes alloys. The term cylinder comprehends cylindric shapes having any useful cross sections including, without limitation, circular, ovate, polygonal, crescent and petaliform. Circular cross sections are shown and described for disclosure-purposes. By work-hardening a metal is meant herein the deformation of metal crystals by mechanical means at a temperature below the recrystallization temperature of the metal.- This places the deformed crystals in a higher energy state (thermodynamically) and this energy, according to the invention, is subsequently released during certain operating steps.

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geously utilized for the manufacture of cored wires, rods or the like of various cross sections, yielding a product in which the core and its cover or sleeve are strongly bonded without the interposition of a liquid-phase or brittle intermetallic compound appearing in the bonding area between the parts. The bond obtained in the end product is substantially continuous and strong enoughto withstand severe flexing, stamping and drawing operations. 'The required components are (1) a malleable sleeve or shell and (2) an interfitting malleable cylindrical core which is usually preferably solid but which, if desired, may be hollow. The outside shape of the. core and the corresponding inside shape of the shell may be such that the parts can' readily be telescoped, in which case a slight clearance is provided, for example, several thousandths of an inch. Alternatively, the shell may be extruded in position around the core.

Prior to bringing the components together as above described, they are cleaned so as to remove gross contaminants. This is accomplished by conventional cleaning operations such as brushing, wiping, chemical cleaning, etching, pickling or the like, to remove grease, dirt, grit and similar particles and deterrents to subsequent bonding. It is not necessary that the cleaning shall be as meticulous as that referred to, for example, in Boessenkool et al. U.S. Patent 2,691,815; that is, it is not necessary to remove barrier films which approach molecular dimensions in thickness, such as hydrated or other natural oxide and compound films, chemisorbed layers or adsorbed layers of liquids and gases.

Fig.1 illustrates a first general form of the invention, specific cases of which are itemized as Nos. 1-6 of Table A below. Numeral 1 indicates a circular inner cylinder or core, the outside surface of which has been sufficiently cleaned as described above and which has been introduced into a cylinder or sleeve 3, the inside surface of which has likewise been cleaned. If these components or parts 1 and 3 have been brought together by an extrusion or like process, the surfaces 'may be close enough together to proceed with succeeding steps, since then there is close contact at an interface 5. If they are to be brought together by telescoping, a preformed core 1 is inserted into a preformed sleeve 3 with sufiicient clearance to allow sliding. In the latter case the adjacent surfaces after cleaning and telescoping are brought into good contact with each other by external hammering or rolling, for example. The amount of hammering or rolling is such as to effect good physical contact between the core and the shell at an interface 5. This is the condition of parts shown in Fig. 1, which is fairly illustrative of the dimensional relationships specified under item 2 of Table A for a core of copper surrounded by a sleeve of A181 #304 stainless steel. However, the materials constituting core 1 and sleeve 3 may be selected from any of the various solid malleable metals, provided the metal selected for the core 1 has a coeflicient of thermal expansion which is greater than that of the metal selected for the sleeve 3. This is the case for the first six cases in Table A.

It is also required that the material of one or both of the members 1 and 3 shall have been or shall be workhardened at the time that these members are organized as shown in Fig. 1, and that this work-hardening shall exist at the interface 5. The above-mentioned rolling or hammering operation, which brings about the interfacial contact at 5, may contribute to or be sufficient to produce the word-hardening desired, but if not, the work-hardening may be provided before assembly on either or both pieces by work-hardening operations, such as drawing, breaching, extrusion or the like. Whatever operation is employed for work-hardening shouldminimize the deposition of those contaminants that cannot readily be removed by conventional cleaning operations and should not heat the surfaces to such a degree as to interfere with the work-hardening desired.

, It will be understood that, although both of the layer faces to be interfacially contacted may be work-hardened, the process may be carried out with only one of said faces so work-hardened, the other being of soft temper, i.e., not work-hardened. This may be necessary in the event that one component has a recrystalization temperature above the melting point of the other component. The cleaning step .as described should succeed the work-hardening step, unless the latter results in a sufficiently clean surface. Preferably, in the case of a work-hardened rod and tubing employed as raw material, the cleaning step will succeed work-hardening, unless the sleeve is extruded onto the core.

After one or both pieces land 3 have been suitably work-hardened, both cleaned, assembled and contact effected at the interface as above described and as shown in Fig. 1, they are heated to bring about a first partial bonding of the components at said interface. During this heating step pressure is generated between the contacting surfaces at the interface 5, due to the differential expansion between the core 1 of relatively large thermal expansion coeflicient and the shell 3 of relatively small thermal expansion coefficient. Thus there is simultaneously ac- .complished an elevated temperature and a pressing together of the metals at the interface 5, without the use of external forcing means during the heating. Thus the application of heat has a dual function, namely, to press the surfaces together at the interface 5 and to raise the temperature at the interface which, in view of the prior work-hardening of at least one piece at the interface, will rapidly effect some diffusion bonding across the interface.

The above-mentioned partial bonding by diffusion is accompanied by a lowering of the higher energy state which was produced by the mechanical working. To this end a temperature is brought about by heating whichlies within the range which extends from and includes the recrystallization temperature of the one of the metals of elements 1 and 3 which has been work-hardened, or if both metals have been work-hardened, then from and including the recrystallization temperature of the one having the highest recrystallization temperature, and in either ease up to but not including the temperature at which a liquid-phase or brittle intermetallic compound would appear at the interfacial area 5. The temperature employed effects rapid diffusion across the interface during or near recrystallization, accompanied it is believed by said release of at least some of the thermodynamic energy bound up in the work-hardening condition of one or both metals at the interface. Thus is created conveniently a first partial bond between the metals throughout the interface 5. A range of temperatures within substantially 400 F. to 1850 F inclusive, will cover the temperatures required in this step in many cases, in order to bring about rapid diffusion in a relatively short time. As cases 1-6 of Table A indicate, the higher temperatures are employed in order to reduce the partial bonding time to a matter of minutes, or less.

In explanation of the partial bonding effect, it may be noted that the pressure engendered between the surfaces at the interface 5 by heating and differential expansion brings about a multitude of discrete contact points. Each is under pressure and across each the stated diffusion takes place. Thus at each contact there results a substantial bond, but since the contacts, multitudinous as they are, do not entirely cover the interfacial area, a partial or incipient bond only occurs between the metals, in the sense that the integrated interfacial bond is not as strong as if the metals were, for example, continuously welded by a liquid phase throughout the interfacial area. Therefore the partial or incipient bond generated in the stated first heating and pressing step is relatively weak, but sufficient to prevent separation under manipulation according to subsequent steps to be carried out. For example, cooling will not separate this partial bond, nor will coiling, looping or similar treatment normally to be expected in passing through the subsequent steps of the process.

The time required for the above initial heating and pressing to produce an initial partial bond is somewhat variable, but a suitable range appears in Table A. The time should be sufficient to assure obtaining all of the partial bond strength desired by means of the simultaneous heating and pressing step. It will be understood in this connection that the final increased bond effected by our method is not to be obtained during the partial bonding stage. On the other hand, while a substantial portion of the partial bond is obtained in the first few minutes of heating at the correct temperature, some additional time may be allowed for the fact that uniform application of the required temperature to all of the composite rod or wire mass may require some time. Increase in the heating time favors such uniformity. In some instances this heating time for partial bonding may be as low as 10 minutes at some temperatures within the required temperature range as above given.

Heating for partial bonding may be accomplished by any known means, such as by a surrounding conventional type of protective atmosphere furnace (not shown). The protective atmosphere is sometimes desirable, so as to preserve exterior finish, or in connection with certain metals which oxidize readily, such as beryllium copper, for example. In general, however, and except in rare cases, the protective atmosphere is not necessary insofar as the contact faces of 1 and 3 at the interface 5 are concerned.

In certain cases it may be desirable in order to avoid subsequent stretch-off to seal the ends of the shell 3 around the ends of the core 1 after the surfaces have been cleaned and placed in contact. This sealing may be accomplished by pinching off the ends of the shell 3, as indicated at 7 in the variation shown in Fig. 2, and Welding these ends as indicated at 9. With these ends so sealed, the simultaneous heating and pressing step can then in most cases be carried out in a nonprotective atmosphere. By use of this variation, stretch-off during subsequent mechanical reduction will also be minimized. However, even when this variation is employed a pro tective atmosphere will be required in a few cases, as, for example, when it is desired that the outer surface of sleeve 3 should not be corroded, oxidized or otherwise affected by the hot ambient atmosphere. Another case occurs, for example, when a thin shell of silver is to be bonded to a titanium core wherein the silver might oxidize all the way through to the core and prevent the formation of a-satisfactory bond.

In the cases in which protection against oxidation is not required, the enclosures shown in Fig. 2 are not necessary except to avoid the stated stretch-off. Moreover, such enclosures are not required to avoid stretchoff Where the core material 1 is less malleable than the shell material 3, for in such case the core materials will not be squeezed out beyond the sleeve material during the subsequent mechanical reduction. In this case it might be desirable to make the core longer than the sleeve.

Next, the partially bonded assembly is mechanically reduced, usually after it has cooled, since some time is ordinarily required to bring the partially bonded material from the station at which partial bonding is accomplished to the station at which mechanical reduction is to be ac.- complished. However, cooling is optional. Mechanical reduction or squeezing may be accomplished by drawing the partially bonded assembly through a conventional drawing die at a temperature below the recrystallization temperature of that component'thereof having the highest recrystallization temperature, provided that this temperature is below that at which a liquid phase would appear at the interface or at which brittle intermetallic compounds would form. Reduction is carried out in one or more passes, so as to avoid disruption of the partial bond that might occur becauseof excessive differential 7 Table 4 Material 1 Sizes Before Bonding a I Reduc- Temp.

1 1 Outside Partial. Partial tion Final Case Outside Inside Jacket Diamrat, Bonding, Bonding, after Heating N051 s I Diam. Diem. Wall Bonding, Tempi, 'Ijimel." Partial. Step, 5 Core. Shell Shell, Shell, Thickinches 3,111;- min. Bonding, 2B

' inches inches ness, inches mches i .Nickel 0. 750. 0.000 .700 1,000 .550 1,300 AISSfiI #1446 Stainless 1.200 0. 500 1.100 1,550 120 1.050 1,550

. 1 AISI #30 4 Stainless 1.500 0.862 1.460 1,850 120. 1.437, 1,850 flgilwhite Gold 1. 300 1.140 1. 200 1, 500 45 1. 220 1, 300 oyfi'" .Sc#20.B,1jass 1.300 .760 1.255 1,420 45 I 1.160 1,200 I 65,'35B rass- 1.300 385 1,325 45 1,160 1,200

.Coppernu; 1.372 1.300 .100 1.412 1,550 60 1,295 1,300 "mfnYe lljow Gold 1.326 1,300 .075 1.4 1, 550 60 1.295 1,300

- .150 An,- 0.11, (1211, 15 Ni.

I Tab e A. he rsilust qn s r for cases 7 and 8 on the basis of the jacket 17bein g in place around pieces nai -1. a

As Table A shows, the reductions of diameters eftented are relatively small, compared with the'redu c tions in thicknesses specified in said application Ser. No. 012,035,510 in sa d P int 2. This is because given diametral reduction percentages on core and sleeve composites result in greater interfacial area increases. than corresponding interracial area increases. afforded by. the same rednct-igns applied to composite sheets by rolling. Thus the. percentage oi? reduction on a diametr-al basis. according to our invention may be smaller than ten percent and on the other hand as high as may be without destroying any partial bond obtained.

. Broadly, the invention comprises the establishment of a simultaneous heating and pressing operation for partial bonding purposes upon a coaxial assembly of coreforrning and sleeve;forrr 1ing elements, which operation for the i t acia pr s u e end d depends nly p stre se intern l y hh ehde ed b the h n o the sem l-n hus is. axqid d h n h y for usingv external heating presses andthe like for effecting simultaneous. heating and pressing.

In theform ot the inventionshown in Figs. 1 and 2, the internal stresses in the assembly are generated as a. result of; the selection of thematerials for the assembled core and sleeve such that the sleeve has a lower coeflicient otthermal expa sion han t e q n e f o t invention shown in Figs. 3 and 4, the stresses are a result of selection of alower. coetiicient of thermal expansi n tor t leas an. ou er pa 1 of What y referred to as a composite sleeve, having outer and inner parts 17; and 1 3 these parts being discrete. While ordinarily. only. e inner par 13 ined a a eev on the final product, both parts may under certain conditions be s re ained, asabove ad a In view. of the above, it will be seen that the several objects of the. invention are achieved and other advant g ousres lt ta ned- As, various changes couldbe made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above descrip tion or shown. in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

1. The method of solid-phase bonding core-forming and sleeve-formi g. lem n s omp sed 0f malleable metal terials, which elements are cleaned throughout their surfaces to beinterfacially" bonded and which have at least one. of'said surfaces, work-hardened; comprising coaxially surrounding said. coreri'orming element with sleeve-form iug'.materialrcomppsedQti ne nd u er-p q s. t as the outer portion having a lower coefl'lcient of; thermat n an on than that f e ormin m and establishing substantially coextensive interfacial contact;

between the outside of'said core-forming element and;

I the inside of said sleeve-forming element, heating the,

assembly ,at a temperature lying within the range which extends. from, and includes. the recrystallization tempera -v ture. of the. one. of the metals to be bonded which has been work-hardened, or if; there be more than one work hardened metal to be bonded, then. from and including the.

' recrystallization temperature of the one which has they highest recrystallization temperature, and in either case partial bond between the core-forming and sleeve-forming.

elements, squeezing at least the inner portion of: the. sleeve-forming element with the partially bonded coreforming element therein at a temperature below the re.-. crystallization temperature of that one of the partially. bonded metals having the highest recrystallization ternperature', said squeezing. being. carried out with areduc tion of the cross-sectional areas of the coreand sleeveforming materials adapted to produce a concomitant substantial increase in the bonding area, and heatingrthe metals at a temperature which will recrystallize at least one of' the partially bonded metals, but below one of those temperatures, whichever is the lower, as the case may be, at which a liquid-phase material or brittle intermetallic compound would form in appreciable amount, said heating being continued for such a time as to effect substantial growth of the partial bond, thereby substantially to increase the over-all bond strength.

2. The method according to claim 1, wherein the inner and outer portions of the sleeve-forming element are formed by discrete inner and outer sleeves, said inner sleevefhaving a coefiiciento f thermal expansion which is no less than that of said core-forming element.

' 3. The method according to claim 1, wherein the inner and outer portions of the selceve-forrning element, are formed by discrete inner and outer sleeves, the inner sleeve being sufiiciently thin and the outer sleeve sulficiently thick that the inner sleeve is constrained by said; outer sleeve to be contacted under pressure by the coreforrning element when heated, regardless of thevalue of the coeflicient of thermal expansion of the inner sleeve, whereby the inner sleeve and the core-forming element become interfacially pressed together during heating.

4. The method according to claim 3, wherein prior to.coaxial assemblya parting material is located between said innerv and outer sleeves, and wherein the outer sleeve is removed after said heating steps have been performed. 5'The 'niethod of solid-phase bonding a core and slee' composedof. malleable metal materials which arev 'ltaa d. hi ss sut e r a i-ash to. be inter-Wally;

amass work-hardening of the core 1 and shell 3. During each pass some work-hardening occurs, as is desired, since this again raises the potential energy available at the interface preparatory to a final heating to be described, although without disruption of the previously formed partial bond. In cases of metals of widely different work-hardening characteristics, several reduction passes are preferred to obtain the total reduction desired.

During the drawing operation the outside diameters of the core 1 and sleeve 3 become reduced while they elongate. Thus, although the diameter of the cylindrical interface 5 becomes less, it becomes longer to such an extent that there is a concomitant increase in the interfacial area 5. The amount of reduction per pass for a given pair of metals should be as much as can be tolerated without disrupting the partial bond. The total amount of reduction has a substantial range, as is apparent from Table A.

In some cases it may be desirable to keep the metals at an elevated temperature after the partial bonding step in order to control the workability in the reduction step.

The last step in the process after said reduction step is a final heating step for the purpose of substantially increasing the :bond strength of the previously produced partial bonds by lateral growth and diffusion during or near recrystallization. This increased bond strength is sufficient to supply all of the bonding strength needed in the final product. The final heating should be at a temperature which will recrystallize at least one of the metals. The time employed is of an amount to provide a sufficiently perfected bond and will in general be on the order of minutes to an hour or so. Suitable temperatures are shown in Table A.

In some cases it may be desirable to solid-phase bond a combination of shell and core material wherein the shell has a coefficient of thermal expansion equal to or higher than that of the core; and/or the wall thickness of the shell is comparatively thin with respect to the diameter of the core, such as for example, a Wall thickness of 0.08" and a core diameter of 1.50". Under such conditions it will be difficult to obtain the necessary pressure during the first heating step for partial bonding. Under such circumstances, a second embodiment of the invention and :a variation may be employed, such as illustrated in Figs. 3 and 4 respectively, examples of which are constituted by cases 7 and 8 of Table A.

Numeral 11 in Fig. 3 illustrates a cylindrical core and numeral 13 a thin sleeve, both of which have been cleaned as above described and brought into coaxial relationship. The core 11 (nickel) has a lower coefficient of expansion than the sleeve 13 (copper). It will be understood that one or both cores 11 and sleeve 13 have been work ha-rdened and cleaned in the manner described above in describing the work-hardening and cleaning of the core 1 and sleeve 3 of Fig. 1.

The core and sleeve are also surrounded by a coaxial steel jacket or sleeve '17, which has a eoeflicient of expansion lower than that of the nickel core 11. Clean surfaces between the jacket 17 and sleeve 13 are unnecessary. In some cases it may be desirable to apply a parting compound such as graphite or milk of magnesia between the jacket 17 and sleeve 13. The jacket may or may not be work-hardened. After the core 11, sleeve 13 and jacket 17 are brought together, the combination is externally hammered or rolled to bring about good contact at the interface and also at the interface 19 between the sleeve 13 and the jacket 17. The core 11 and sleeve 13 assembly may be externally hammered. or rolled prior to application of the jacket -'17 and ham- .mering or rolling thereafter completed.

Next, the parts I11, 13 and 17 are subjected to the above-described partial-bonding heating and pressing step for engendering pressure at the interface 15 and bringing about rapid diffusion across the interface 15. Upon heating, pressure occurs across the interface even though the sleeve 13 has a coefiicient of expansion equal to or higher than that of the core 11, and/or is too is provide by itself sufiicient hoop strength to engender the desired pressure. This is for the reason that the steel jacket 17, having a coeflicient of expansion lower than the core 11, confines the sleeve 13. Thus the core 11,

, upon heating, through temperature rise effects a radial squeezing action upon the sleeve 13, resulting in the desired pressure at the interface 115. The heating is accomplished under temperature conditions above specified in connection with the heating for partial bonding of core 1 and sleeve 3 in Fig. 1, and as indicated more particularly under cases Nos. 7and 8 of Table A. These resulting partial solid-phase bond has been establishedat the interface 15, the assembly shown in Fig. 3 may be allowed to cool, whereupon jacket 17 may be removed from the sleeve 13, sleeve 13 being then partially bonded to the core 11. This removal may be accomplished in any of various manners, such as, for example, skiving off the jacket 17; slitting and peeling it; or removing it chemically by dissolution.

It will beunderstood that if, it were desired to have core 11 with several sleeves thereon, such as for example the core 11, sleeve 13 and jacket 17 (considered as a final sleeve), this could be accomplished by preliminarily also cleaning the surfaces between members 13 and 17. In such event, provision would be made for work-hardening at least core 11 and jacket 17, or work-hardening at least the shell 13; this in order that all three parts 11, 13 and 17 will bond together. In such case a partial bond would be established during the first heating step not only between the parts 11 and 13 but also between parts 13 and 17, thus providing as the end product a double-sleeved core. In this case, of course, no parting compound would be applied between parts 13 and 17.

It is preferable that the wall thickness of the shell 13 be small relative to its diameter (Fig. 3), in order that its expansive force will not crowd the jacket 17 outward and overcome the constrictive effect of the latter. It will be understood, however, that its thickness is somewhat variable, depending upon the metals used and the thickness of the jacket 17.

The next step is the reduction step by pulling through drawing dies. This reduction step may be performed upon the assembly of Fig. 3 as a whole, whether or not the jacket 17 is to be bonded to the sleeve 13. Or, if preferred, in case the jacket 17 is not to be bonded the partially bonded assembly 1 1, '13 may be removed from jacket 17 and reduced. Appropriate reductions with the jacket 17 in place during reduction are shown, for example, in cases 7 and 8 given in Table A.

Finally, the partially bonded parts 11 and 13 of the invention shown in Fig. 3 are heated after removal of the jacket '17, to increase the bond strength between 11 and 13, as above described in the case of the Fig. 1 form of the invention; or they may be likewise heated before removal of the jacket 17, whether or not bonding between all three elements 11, 13 and 17 isprovided for by proper cleaning between pieces 17 and 13.

Referring now more particularly to Fig. 4, it will be seen that after assembly of the core 11 sleeve 13 and jacket 17, and before heating, the assembly may be pinched 01f at the ends, as shown at 21, and welded as indicated at 23. The circumstances under which this protective arrangement is employed are the same as those referred to in connection with the Fig. 2 modification of the Fig. 1 embodiment. I

Following is Table A, in which cases Nos. l-6 are processed in accordance with the description concerning Figs. 1 and 2, and in which cases Nos. 7 and 8 are processed in accordance with the description concerning Figs. 3 and 4: r Q

bonded and which have at least one of said surfaces work-hardened; comprising coaxially surrounding said core with sleeve-forming material composed of metal having a lower coefficient of thermal expansion than that of the core, establishing substantially coextensive interfacial contact between the outside of said core and the inside of the sleeve, heating the contacted assembly at a temperature lying within the range which extends from and includes the recrystallization temperature of the one of the metals to be bonded which has been work-hardened, or if there be more than one work-hardened metal to be 'bonded, then from and including the recrystallization temperature of the one which has the highest recrystallization temperature, and in either case up to but not including the temperature at which a liquid-phase material or brittle intermetallic compound would appear in the bonding area, in order to create a first partial bond between the core and sleeve, squeezing the sleeve with the partially bonded core therein at a temperature below the recrystallization temperature of that one of the partially bonded metals having the highest recrystallization temperature, said squeezing being carried out with a reduc-- tion of the cross-sectional areas of the core and sleeve materials adapted to produce a concomitant substantial increase in the bonding area, and heating the metals at a temperature which will recrystallize at least one of the partially bonded metals, but below one of those temperatures, whichever is the lower, as the case may be, at which a liquid-phase material or brittle intermetallic compound would form in appreciable amount, said heating being continued for such a time as to effect growth of the partial bond, thereby substantially to increase the over-all bond strength.

References Cited in the file of this patent UNITED STATES PATENTS 2,267,665 Raydt et a1. Dec. 23, 1941 2,371,348 Murray Mar. 13, 1945 2,753,623 Boessenkool et a1. July 10, 1956 2,834,102 Pfiumm et al May 13, 1958 

